Porous filters made of polytetrafluoroethylene (PTFE) have characteristics of PTFE, such as high heat resistance, chemical stability, weather resistance, a fireproof property, high strength, non-adhesiveness, and a low friction coefficient, and also have characteristics of a porous material, such as flexibility, liquid permeability, particle retention, and a low dielectric constant. Therefore, porous filters made of PTFE have been widely used as microfiltration filters for liquid and gas in the semiconductor field, the liquid crystal field, and the food and medical treatment fields.
In such fields, because of further technological innovation and increasing requirements, there has been a demand for microfiltration filters having higher performance. Specifically, in the semiconductor field and the liquid crystal field, the degree of integration has been increasing year by year, and photoresists have been used in fine regions with a width of 0.5 μm or less. Therefore, there is a need for microfiltration filters that can reliably retain such fine particles. These microfiltration filters are used mainly as filters for treating the outside air in clean rooms, filters for filtration of chemical solutions, and the like, and their performance affects the yield of products. Furthermore, in the food and medical treatment fields, along with recent increases in safety awareness, there has been a strong demand for filters having a high removability of very small pieces of foreign matter.
In order to meet such demands, a porous filter in which a porous sheet made of PTFE capable of retaining fine particles with a particle diameter of less than 0.1 μm is used has been proposed (refer to Japanese Unexamined Patent Application Publication No. 2010-94579).